Add configuration items to limit secret and request sizes.
Add configuration items to limit secret and request sizes.

Two size attributes limit the size of secrets and requests in barbican. This CR
explicitly adds those to the Barbican configuration file.

Change-Id: I22d68cc8c9e96cf719a550b26333a845e895f178

cloudkeep/barbican,cneill/barbican,openstack/barbican,jmvrbanac/barbican,cneill/barbican,MCDong/barbican,jmvrbanac/barbican,cloudkeep/barbican,MCDong/barbican,openstack/barbican